Variable displacement refrigerant compressors have been employed in engine driven automotive air conditioning systems in order to reduce engine load variations associated with compressor cycling. In the system manufactured by the Harrison Radiator Division of General Motors Corporation, for example, the compressor displacement is controlled by regulating the compressor crankcase pressure. To this end, a pneumatic control valve integral to the compressor variably connects the compressor crankcase to the inlet (suction) and outlet (discharge) chambers of the compressor.
In an electronic version of the control, the control valve is mechanized with a solenoid valve positioned to achieve the ratiometric control. In one mechanization, the valve may be pulse-width-modulated at a variable duty cycle to alternately connect the crankcase to the inlet and outlet chambers. In another mechanization, the valve may be linearly positioned by controlling the solenoid current, a direct analog to the pneumatic control referred to above.
With any such control, the objective is to adjust the crankcase pressure as required to achieve a desired system condition, such as a desired system temperature or pressure. However, nonlinearities inherent in the operation of a variable displacement compressor make it difficult to achieve an accurate and stable control. For example, the control gain (which may be defined as the change in compressor displacement per unit change in PWM duty cycle in the case of an electronic control) varies with internal friction, ambient temperature and the system loading. This means that the variation occurs not only from compressor to compressor, but also in a given compressor installation with wear, ambient conditions and control setting.